Ch 48 Support Systems and Locomotion
Diversity of
Skeletons
Hydrostatic Skeletons
How does
a hydra that has no hard skeleton extend itself rigidly, bend, and move? Skeletal Muscles contract thereby bringing about
movement, and skeletons are generally thought of as compression
resistant structural adaptations that give support to skeletal
muscles. Skeletons serve as a
supporting structure for muscles. The
support structure of a skeleton we usually think of as bone in our body,
yet this is only one type of supporting structure that enable muscles to
function in bringing about movement.
The supporting skeleton can be a fluid filled chamber that
resists compression when squeezed upon, i.e. a hydrostatic skeleton.
While the fluid is not compressible it is very capable of being
molded into different shapes. Thus,
when the layer of circular muscles contract in the body stalk of hydra,
the entrapped hydrostatic fluid held within the gastrovascular cavity is
forced upward forming an elongate column supporting the elongated trunk of
the hydra. When longitudinal
muscle cells in the hydra’s stalk contract, the fluid residing in the
gastrovascular cavity is forced downward and outward creating a short,
wide stalk. The hydra moves
and can stand erect if needed. In earthworms (earthworms are annelids) the
hydrostatic skeleton is more complex due to the segmentation of the body.
The trunk of hydra works as one unit, either elongating or
shortening as a whole. In an
earthworm, each segment is partitioned off internally from the neighboring
segments and to some extent works independently. An earthworm’s muscles are arranged in segmental units
within the body wall and the body cavity is also partitioned between
segments internally. Thus, a
worm can anchor all but its very front end to the ground using extendable setae
(bristles that protrude from the worm) and work the hydrostatic skeleton
of the head end to actively burrow using its front to push soil particles
aside. The action of the muscles on the hydrostatic skeleton of the
worm is essentially the same as in hydra:
circular muscles contract forcing the hydrostatic fluid into a
narrow column thus elongating the body of the worm; longitudinal muscles
contract forcing the hydrostatic fluid into a short, wide cylindrical
shape supporting a short, wide worm.
The difference lies in the repeating units of hydrostatic skeleton
as found in the earthworm. Thus,
different segments can perform different movements simultaneously. Hydrostatic fluid – is water taken in from surrounding environment and held in the gastrovascular cavity (e.g. hydra and sea anemones). Hydrostatic
fluid – is body fluid found in the body cavity (coelom), thus
coelomic fluid (e.g. earthworm). Hydrostatic
skeletons are found in soft-bodied invertebrates including in addition to
the annelids and cnidarians mentioned above, mollusks (snails, squids,
etc), flatworms (planarians), nematodes, and other lesser groups. Exoskeletons Exoskeletons
are of varied form. In
general, they are hardened body coverings that provide physical protection
to softer body parts within. Exoskeletons
also provide sites for muscle attachment.
Examples
of Exoskeletons
Exoskeletons
as a type of full body armor limit the size of the animal due the weight
of the exoskeleton. Endoskeletons
Vertebrates have an endoskeleton of cartilage in the case of sharks, most other vertebrate have an endoskeleton of bone and cartilage. Features of the human endoskeleton will be covered in a lab following spring break. |